1. Field of the Invention
The present invention relates to a liquid discharging head and a liquid discharging apparatus which include a plurality of common passage members that define common passages communicating with all liquid chambers in head chips and which discharge liquid from the liquid chambers by driving energy generating elements of the head chips so as to apply a discharging force to the liquid in the liquid chambers, and to a bubble removing method for the liquid discharging apparatus. More particularly, the present invention relates to a technique of stably supplying liquid to the common passages of the common passage members and of smoothly removing bubbles from the liquid in the common passages.
2. Description of the Related Art
In an inkjet printer as an example of a liquid discharging apparatus, a recording sheet is conveyed to a liquid discharging head, and ink (liquid) is discharged for printing on the recording sheet by driving a heating resistor (energy generating element) in an ink chamber (liquid chamber) of a head chip that constitutes the liquid discharging head. In this inkjet printer, it is necessary to stably supply ink stored in an ink cartridge to the ink chamber of the head chip.
Water-based ink and oil-based ink can be discharged. Particularly when water-based ink is used, air dissolved in the ink sometimes form bubbles, for example, because of a temperature change, or air taken from the outside sometimes remains as bubbles in the ink. If these bubbles accumulate near the head chip, the flow of ink to the ink chamber is hindered, and sufficient ink supply is difficult during printing. For this reason, the bubbles in the ink disturb the ink discharging direction and change the ink discharging amount.
Ink is discharged by the application of a discharging force from the heating resistor in the head chip to the ink in the ink chamber. If a bubble exists in the ink, it weakens the ink discharging force because of gas compressibility, and disturbs the ink discharging direction. Further, if the bubble in the ink is expanded in accordance with the installation environment of the inkjet printer, the temperature change due to ink discharging (driving of the heating resistor), or the change in atmospheric pressure, the ink in the ink chamber is sometimes unintentionally discharged from the nozzle.
In order to overcome the above problems due to the existence of bubbles in the ink, various technologies for removing bubbles from the ink have been proposed. For example, in the case of a line printer which performs printing corresponding to the width of the recording sheet with nozzles arranged over the length corresponding to the width of the recording sheet, the number of prints is large and good durability is necessary. For this reason, bubbles near the head chip are removed from the ink by being circulated together with the ink by a transfer means such as a pump.
In this bubble removing method, bubbles can be removed with the flow of ink in the common passage communicating with all ink chambers of the head chip. That is, ink is ejected from an outlet of a buffer tank (common passage member) that defines the common passage while supplying ink from an inlet of the buffer tank, so that bubbles are removed from the common passage together with the ink.
However, when in a module-type line head (line printer) that can perform printing corresponding to the width of a recording sheet with a plurality of head modules each including a head chip and a buffer tank, there is a problem with a method for supplying and ejecting ink to and from each buffer tank. That is, it is necessary to remove bubbles from the ink in common passages of all buffer tanks. The ink can be supplied to and ejected from the common passages by some routes.
In one bubble removing method using parallel connection, an ink supply member is connected to an inlet of each buffer tank so as to directly supply ink to common passages of all buffer tanks, and ink is directly ejected from an outlet of each buffer tank. For example, Japanese Unexamined Patent Application Publication No. 11-342634 discloses a line printer in which inks in common passages of all buffer tanks are circulated in parallel by one pump so that bubbles are simultaneously removed from all the common passages.